Because MOST people under the age of majority have not acquired enough maturity and responsibility to safely handle firearms without supervision. Of course, age is no guarantee that that has happened, but does make it more likely.

Most? really? Is this mere anecdote or do you have hard data to back that up?

Let's take another look at what you stated, but substitute other terms to it:

Because MOST people under the age of majority have not acquired enough maturity and responsibility to safely handle firearms manage their own finances/vote/manage a family/drive/drink/serve their country (insert any one of those or insert your own) without supervision. Of course, age is no guarantee that that has happened, but does make it more likely.

Sorry, this is a Brady tactic: Conflating Some or Most for Many.

Do you really think that the fundamental right to self protection can be legislated away, because of an arbitrary age of majority? An age, I might remind you, that has already been lowered to 18 in many, if not most, all other aspects of your life.

If you seriously do think this way, then what other civil rights should we take away from these infants?